Engine out cold-start emissions generated before light-off of an exhaust system catalytic converter may contribute a large percentage of the total exhaust emissions. Various approaches may be used by engine control systems to expedite the attainment of the catalyst light-off temperature.
One example approach, disclosed by Surnilla et al. in U.S. Pat. No. 6,978,204 relies on significant ignition timing retard on one engine bank to produce significant amount of heat and little engine torque output during an engine cold-start condition. At the same time, the other engine bank is run slightly leaner than stoichiometry, and with spark timing slightly retarded from maximum torque timing to maintain the cylinders at a desired engine speed. By running significantly retarded spark on one bank, the airflow needed to maintain idle torque is increased while the combustion is phased late. As a result, heat is released into the exhaust system, getting the catalyst up to operating temperature more quickly. Still other approaches may include the use of transient enrichment, elevated engine speed, or a combination of increased fuel injection and spark timing retard to expedite catalyst warming.
However the inventors herein have recognized that there may be limitations to the amount of spark retard and air flow that can be obtained. For example, if the spark timing is overly retarded, engine combustion may become unstable. As such, this may lead to undesirable NVH effects.
In recent years, engines have been configured to operate with a variable number of active or deactivated cylinders to increase fuel economy, while optionally maintaining the overall exhaust mixture air-fuel ratio about stoichiometry. Such engines are known as variable displacement engines (VDE). Therein, a portion of an engine's cylinders may be disabled during selected conditions defined by parameters such as a speed/load window, as well as various other operating conditions including vehicle speed. A VDE control system may disable a selected group of cylinders, such as a bank of cylinders, through the control of a plurality of cylinder valve deactivators that affect the operation of the cylinder's intake and exhaust valves, or through the control of a plurality of selectively deactivatable fuel injectors that affect cylinder fueling. Further improvements in fuel economy can be achieved in engines configured to vary the effective displacement of the engine by skipping the delivery of fuel to certain cylinders in an indexed cylinder firing pattern, also referred to as a “skip-fire” pattern.
The inventors herein have recognized that the limitations pertaining to the use of spark retard during cold start conditions may be overcome by leveraging skip-fire operation of engine cylinders. In one example, the issues may be addressed by a method comprising: during an engine cold start, operating with a number of cylinders selectively deactivated and spark timing of remaining active cylinders retarded by an amount; and increasing engine speed to maintain an idle torque, the engine speed increased based on the number of deactivated cylinders. In this way, more spark retard may be tolerated in the active cylinders to expedite catalyst heating while the engine speed may be increased to address NVH issues.
As an example, during an engine cold-start, an engine with individual cylinder valve mechanisms may be operated with a cylinder pattern where a number of cylinders are selectively deactivated. The number and identity of individual cylinders valve mechanisms that are deactivated and a remaining number of engine cylinders that are maintained active may be based on the engine temperature at the cold-start. The active cylinders may be operated with spark timing retarded from MBT. As such, since the remaining active cylinders are operated at a higher average cylinder load to produce the same net torque output as with all engine cylinders firing, the active cylinders may be able to tolerate a higher amount of spark retard before combustion stability limits are reached. Thus, spark may be retarded in the active cylinders by a higher amount than would otherwise be possible with all cylinders running. To also address any NVH issues arising from remaining active cylinders firing at different overall frequencies, and with larger input (torque) pulses, the engine speed applied to maintain engine idle torque may be adjusted. For example, the engine speed may be increased as the number of firing cylinders decreases while operating the engine with a given amount of spark retard.
In this way, based on the heat flux required during an engine cold-start, various combinations of spark retard, cylinder deactivation (number and pattern), and engine speed adjustment may be applied to expedite catalyst heating without incurring NVH issues. The various combinations may be arranged in look-up tables and used in an open-loop fashion, or closed-loop based on exhaust temperature. The technical effect of deactivating individual cylinder valve mechanisms according to a defined skip-fire pattern during an engine cold-start is that the active engine cylinders may be operated with a larger amount of spark retard. In addition, the engine may be operated with one or more cylinders deactivated, thereby allowing for less heat transfer to the coolant since there is less cylinder surface in contact with the coolant. As such, this allows catalyst heating to be expedited. In addition, by controlling the engine speed based on the cylinder deactivation during the cold-start, catalyst light-off may be achieved with improved NVH characteristics. Overall, engine cold-start performance and emissions compliance may be improved.
The above discussion includes recognitions made by the inventors and not admitted to be generally known. Thus, it should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.